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Old 6th March 2012, 07:00 PM   #1
diyAudio Member
Join Date: Sep 2009
Default small PSU using 2 reg. 7808 and LT 3080 (5V)

Hi folks,

I need to rig up a small PSU giving 8V using a 7808 and giving 5V using a LT3080.

I was wondering if I should take the V+ from the 7808 to feed the LT3030, using the 7808 as a pre-regulator or rather feed them both from the unregulated 12V supply.

Benefits / Drawbacks ?

I don't feel like doing a sim on it, so any advice is appreciated.


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Old 6th March 2012, 07:11 PM   #2
diyAudio Member
Join Date: Jan 2012
What current are you going to draw from each voltage.
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Old 6th March 2012, 07:32 PM   #3
diyAudio Member
Join Date: Sep 2009
Well, I'd say almost nothing... if I can say so.

5V will be feeding 2 TORX Toslink receivers and 1 TOTX Toslink emitter, 8V will be used for switchig a source selection relais...

I expect the total current drawn to be under 100mA. If the values are crucial I can dig out the datasheets.


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Old 7th March 2012, 08:59 AM   #4
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Join Date: Jan 2012
Suggest running LT3030 from 12V to share the heat dissipation between the 2 regulators and give max isolation from relay switching .
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Old 7th March 2012, 05:10 PM   #5
diyAudio Member
Join Date: Sep 2009
Thanks a lot for the input.. will proceed further.
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Old 7th March 2012, 10:42 PM   #6
Minion is offline Minion  Canada
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island
I just tried running a Preamp(12vDC regulated with LM317) and a Channel switching curcuit that uses relays (5vDC regulated with LM7805)) from the same 12v AC Transformer and ran into problems right off the bat ...... When I enganged Relay curcuit I would get a pop in the Audio of the pre amp , When I used a seperate transformer for each the pop went away ......

Hopefully you don"t have the same problem ......
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